1. Field of the Invention
The present invention relates to an electrode position control method for tandem arc welding which controls the positions of a leading electrode and a trailing electrode with respect to a weld line (the groove center of a welding workpiece) prior to start of welding, a robot controller for a tandem arc welding system, and a tandem arc welding system.
2. Description of the Related Art
In tandem arc welding in which welding is carried out by using two electrodes including a leading electrode and a trailing electrode, as shown in FIG. 7A, it is necessary to align a leading electrode 2a and a trailing electrode 2b with a weld line previously taught as a welding target, that is, the groove center of a welding workpiece W, prior to start of welding.
With regard to such positional control of the electrodes prior to start of welding, for example, Japanese Unexamined Patent Application Publication No. 2007-307612 proposes a technique which detects the amounts of bending of the two electrodes by a reference jig located at a predetermined position prior to start of welding, and corrects the positions of the two electrodes with respect to an object to be welded (a welding workpiece) by using the detection results of the amounts of bending.
Also, Japanese Unexamined Patent Application Publication No. 54-124850 proposes a technique which performs sensing with a welding torch to determine the relative position of the welding torch with respect to an object to be welded prior to start of welding, and adjusts the welding torch to an appropriate welding position.
However, the technique proposed in Japanese Unexamined Patent Application Publication No. 2007-307612 lacks flexibility in implementation since the reference position can be used only at a predetermined position. Furthermore, although the technique enables detection of the degree of electrode bending, in situations where, as shown in FIG. 7B, the weld line is tilted with respect to the position as previously taught (see FIG. 7A), it is not possible to deal with such situations.
The technique proposed in Japanese Unexamined Patent Application Publication No. 54-124850 is a technique used in single arc welding. Hence, it is difficult to simply apply the technique to tandem arc welding. That is, in tandem arc welding, when a welding workpiece is placed at a position different from a previously taught position due to assembly errors or the like, as shown in FIG. 7B, the weld line is sometimes tilted with respect to the position as previously taught (see FIG. 7A). In this case, as shown in FIG. 7B, although application of the technique proposed in this patent application publication makes it possible to align the leading electrode 2a with the groove center position serving as the weld line, it is not possible to align the trailing electrode 2b with this groove center position.
If welding is started in such a state, the trailing electrode 2d is unable to weld the groove center position of the welding workpiece to be welded, leading to the possibility of a welding defect occurring in the round region bounded by the broken line in FIG. 7B, for example. For cases where the position of the weld groove differs from the position as previously taught (see FIG. 7A) as described above, there has been no technique available to date which corrects the welding start positions of both the leading electrode 2a and the trailing electrode 2b in accordance with the weld line.